Translational
Initiation
Translational Initiation
Protein synthesis initiates by binding of the initiator methionyl-tRNA to the ribosome at an initiation codon. In prokaryotes, initiation requires that the tRNA base pair with an initiation codon, usually AUG or GUG, while the ribosomal RNA (rRNA) base pairs with the mRNA upstream of the initiation codon. The rRNA•mRNA interaction, discovered by Shine & Dalgarno, helps to position the ribosome over correct initiation sites. Prokaryotic ribosomes bind directly to the initiation site, allowing independent initiation of many genes on polycistronic transcripts.
Eukaryotes use a different mechanism. A preinitiation complex of the initiator tRNA, initiation factors and the small ribosomal subunit binds to the 5' end of the mRNA, recognizing among other things a protein complex bound there. The complex moves down the mRNA scanning for an AUG initiation codon. Eukaryotic ribosomes have no Shine-Dalgarno interaction, so they must depend on complementarity with the initiation codon to specify sites of initiation. Not surprisingly, eukaryotes do not normally have polycistronic transcripts since ribosomes can only enter the RNA from one end.
We have found a surprising connection between translational frameshifting and translational initiation. The context sequence that stimulates frameshifting also increases the efficiency of initiation at a non-AUG codon. Initiation at UUG in yeast is normally about 100-fold less efficient than at AUG; the context stimulates UUG initiation about 30-fold. In bacteria, UUG initiation is more efficient, but again the context stimulates UUG initiation, this time to approximately the efficiency of AUG initiation.
Given what we have found about the way that the context stimulates frameshifting, this result implies that disruption of the error-correction machinery by the context affects accuracy of both elongation and initiation. This was a surprise because no data had shown that there was such a connection.
We are continuing to study this phenomenon and hope to determine how the ribosome monitors the accuracy of translational initiation using the context as a tool.